Emacs MIME

This manual documents the libraries used to compose and display
MIME messages.

This manual is directed at users who want to modify the behavior of
the MIME encoding/decoding process or want a more detailed
picture of how the Emacs MIME library works, and people who want
to write functions and commands that manipulate MIME elements.

MIME is short for Multipurpose Internet Mail Extensions.
This standard is documented in a number of RFCs; mainly RFC2045 (Format
of Internet Message Bodies), RFC2046 (Media Types), RFC2047 (Message
Header Extensions for Non-ASCII Text), RFC2048 (Registration
Procedures), RFC2049 (Conformance Criteria and Examples). It is highly
recommended that anyone who intends writing MIME-compliant software
read at least RFC2045 and RFC2047.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being “A GNU Manual”,
and with the Back-Cover Texts as in (a) below. A copy of the license
is included in the section entitled “GNU Free Documentation License”.

(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and
modify this GNU manual.”

1.1 Dissection

The mm-dissect-buffer is the function responsible for dissecting
a MIME article. If given a multipart message, it will recursively
descend the message, following the structure, and return a tree of
MIME handles that describes the structure of the message.

1.2 Non-MIME

Gnus also understands some non-MIME attachments, such as
postscript, uuencode, binhex, yenc, shar, forward, gnatsweb, pgp,
diff. Each of these features can be disabled by add an item into
mm-uu-configure-list. For example,

Emacs source code. This item works only in the groups matching
mm-uu-emacs-sources-regexp.

diff

Patches. This is intended for groups where diffs of committed files
are automatically sent to. It only works in groups matching
mm-uu-diff-groups-regexp.

verbatim-marks

Slrn-style verbatim marks.

LaTeX

LaTeX documents. It only works in groups matching
mm-uu-tex-groups-regexp.

Some inlined non-MIME attachments are displayed using the face
mm-uu-extract. By default, no MIME button for these
parts is displayed. You can force displaying a button using K b
(gnus-summary-display-buttonized) or add text/x-verbatim
to gnus-buttonized-mime-types, See (gnus)MIME Commands section ‘MIME Commands’ in Gnus Manual.

1.5 Display Customization

mm-inline-media-tests

This is an alist where the key is a MIME type, the second element
is a function to display the part inline (i.e., inside Emacs), and
the third element is a form to be evaled to say whether the part
can be displayed inline.

This variable specifies whether a part can be displayed inline,
and, if so, how to do it. It does not say whether parts are
actually displayed inline.

mm-inlined-types

This, on the other hand, says what types are to be displayed inline, if
they satisfy the conditions set by the variable above. It’s a list of
MIME media types.

mm-automatic-display

This is a list of types that are to be displayed “automatically”, but
only if the above variable allows it. That is, only inlinable parts can
be displayed automatically.

mm-automatic-external-display

This is a list of types that will be displayed automatically in an
external viewer.

mm-keep-viewer-alive-types

This is a list of media types for which the external viewer will not
be killed when selecting a different article.

mm-attachment-override-types

Some MIME agents create parts that have a content-disposition of
‘attachment’. This variable allows overriding that disposition and
displaying the part inline. (Note that the disposition is only
overridden if we are able to, and want to, display the part inline.)

mm-discouraged-alternatives

List of MIME types that are discouraged when viewing
‘multipart/alternative’. Viewing agents are supposed to view the
last possible part of a message, as that is supposed to be the richest.
However, users may prefer other types instead, and this list says what
types are most unwanted. If, for instance, ‘text/html’ parts are
very unwanted, and ‘text/richtext’ parts are somewhat unwanted,
you could say something like:

Adding "image/.*" might also be useful. Spammers use images as
the preferred part of ‘multipart/alternative’ messages, so you might
not notice there are other parts. See also
gnus-buttonized-mime-types, (gnus)MIME Commands section ‘MIME Commands’ in Gnus Manual. After adding "multipart/alternative" to
gnus-buttonized-mime-types you can choose manually which
alternative you’d like to view. For example, you can set those
variables like:

When displaying inline images that are larger than the window, Emacs
does not enable scrolling, which means that you cannot see the whole
image. To prevent this, the library tries to determine the image size
before displaying it inline, and if it doesn’t fit the window, the
library will display it externally (e.g., with ‘ImageMagick’ or
‘xv’). Setting this variable to t disables this check and
makes the library display all inline images as inline, regardless of
their size. If you set this variable to resize, the image will
be displayed resized to fit in the window, if Emacs has the ability to
resize images.

mm-inline-large-images-proportion

The proportion used when resizing large images.

mm-inline-override-types

mm-inlined-types may include regular expressions, for example to
specify that all ‘text/.*’ parts be displayed inline. If a user
prefers to have a type that matches such a regular expression be treated
as an attachment, that can be accomplished by setting this variable to a
list containing that type. For example assuming mm-inlined-types
includes ‘text/.*’, then including ‘text/html’ in this
variable will cause ‘text/html’ parts to be treated as attachments.

mm-text-html-renderer

This selects the function used to render HTML. The predefined
renderers are selected by the symbols gnus-article-html,
w3m(1), links, lynx,
w3m-standalone or html2text. If nil use an
external viewer. You can also specify a function, which will be
called with a MIME handle as the argument.

mm-inline-text-html-with-images

Some HTML mails might have the trick of spammers using
‘<img>’ tags. It is likely to be intended to verify whether you
have read the mail. You can prevent your personal information from
leaking by setting this option to nil (which is the default).
For emacs-w3m, you may use the command t on the image anchor to
show an image even if it is nil.(2)

mm-w3m-safe-url-regexp

A regular expression that matches safe URL names, i.e., URLs that are
unlikely to leak personal information when rendering HTML
email (the default value is ‘\\`cid:’). If nil consider
all URLs safe. In Gnus, this will be overridden according to the value
of the variable gnus-safe-html-newsgroups, See (gnus)Various Various section ‘Various Various’ in Gnus Manual.

mm-inline-text-html-with-w3m-keymap

You can use emacs-w3m command keys in the inlined text/html part by
setting this option to non-nil. The default value is t.

mm-external-terminal-program

The program used to start an external terminal.

mm-enable-external

Indicate whether external MIME handlers should be used.

If t, all defined external MIME handlers are used. If
nil, files are saved to disk (mailcap-save-binary-file).
If it is the symbol ask, you are prompted before the external
MIME handler is invoked.

When you launch an attachment through mailcap (see section mailcap) an
attempt is made to use a safe viewer with the safest options—this isn’t
the case if you save it to disk and launch it in a different way
(command line or double-clicking). Anyhow, if you want to be sure not
to launch any external programs, set this variable to nil or
ask.

List of functions used for rewriting the full file names of MIME
parts. This is used when viewing parts externally, and is meant for
transforming the absolute name so that non-compliant programs can find
the file where it’s saved.

We see that the function takes a MIME handle as its parameter. It
then goes to a temporary buffer, inserts the text of the part, does some
work on the text, stores the result, goes back to the buffer it was
called from and inserts the result.

The two important helper functions here are mm-insert-part and
mm-insert-inline. The first function inserts the text of the
handle in the current buffer. It handles charset and/or content
transfer decoding. The second function just inserts whatever text you
tell it to insert, but it also sets things up so that the text can be
“undisplayed” in a convenient manner.

2.2 MML Definition

The MML language is very simple. It looks a bit like an SGML
application, but it’s not.

The main concept of MML is the part. Each part can be of a
different type or use a different charset. The way to delineate a part
is with a ‘<#part ...>’ tag. Multipart parts can be introduced
with the ‘<#multipart ...>’ tag. Parts are ended by the
‘<#/part>’ or ‘<#/multipart>’ tags. Parts started with the
‘<#part ...>’ tags are also closed by the next open tag.

There’s also the ‘<#external ...>’ tag. These introduce
‘external/message-body’ parts.

Each tag can contain zero or more parameters on the form
‘parameter=value’. The values may be enclosed in quotation marks,
but that’s not necessary unless the value contains white space. So
‘filename=/home/user/#hello$^yes’ is perfectly valid.

The following parameters have meaning in MML; parameters that have no
meaning are ignored. The MML parameter names are the same as the
MIME parameter names; the things in the parentheses say which
header it will be used in.

‘type’

The MIME type of the part (Content-Type).

‘filename’

Use the contents of the file in the body of the part
(Content-Disposition).

‘recipient-filename’

Use this as the file name in the generated MIME message for
the recipient. That is, even if the file is called ‘foo.txt’
locally, use this name instead in the Content-Disposition in
the sent message.

‘charset’

The contents of the body of the part are to be encoded in the character
set specified (Content-Type). See section Charset Translation.

‘name’

Might be used to suggest a file name if the part is to be saved
to a file (Content-Type).

Who to encrypt/sign the part to. This field is used to override any
auto-detection based on the To/CC headers.

‘sender’

Identity used to sign the part. This field is used to override the
default key used.

‘size’

The size (in octets) of the part (Content-Disposition).

‘sign’

What technology to sign this MML part with (smime, pgp
or pgpmime)

‘encrypt’

What technology to encrypt this MML part with (smime,
pgp or pgpmime)

Parameters for ‘text/plain’:

‘format’

Formatting parameter for the text, valid values include ‘fixed’
(the default) and ‘flowed’. Normally you do not specify this
manually, since it requires the textual body to be formatted in a
special way described in RFC 2646. See section Flowed text.

Parameters for ‘application/octet-stream’:

‘type’

Type of the part; informal—meant for human readers
(Content-Type).

Parameters for ‘message/external-body’:

‘access-type’

A word indicating the supported access mechanism by which the file may
be obtained. Values include ‘ftp’, ‘anon-ftp’, ‘tftp’,
‘localfile’, and ‘mailserver’. (Content-Type.)

‘expiration’

The RFC822 date after which the file may no longer be fetched.
(Content-Type.)

2.4 Encoding Customization

mm-body-charset-encoding-alist

Mapping from MIME charset to encoding to use. This variable is
usually used except, e.g., when other requirements force a specific
encoding (digitally signed messages require 7bit encodings). The
default is

As an example, if you do not want to have ISO-8859-1 characters
quoted-printable encoded, you may add (iso-8859-1 . 8bit) to
this variable. You can override this setting on a per-message basis
by using the encodingMML tag (see section MML Definition).

mm-coding-system-priorities

Prioritize coding systems to use for outgoing messages. The default
is nil, which means to use the defaults in Emacs, but is
(iso-8859-1 iso-2022-jp utf-8) when running Emacs in the Japanese
language environment. It is a list of coding system symbols (aliases of
coding systems are also allowed, use M-x describe-coding-system to
make sure you are specifying correct coding system names). For example,
if you have configured Emacs to prefer UTF-8, but wish that outgoing
messages should be sent in ISO-8859-1 if possible, you can set this
variable to (iso-8859-1). You can override this setting on a
per-message basis by using the charsetMML tag
(see section MML Definition).

As different hierarchies prefer different charsets, you may want to set
mm-coding-system-priorities according to the hierarchy in Gnus.
Here’s an example:

Mapping from MIME types to encoding to use. This variable is usually
used except, e.g., when other requirements force a safer encoding
(digitally signed messages require 7bit encoding). Besides the normal
MIME encodings, qp-or-base64 may be used to indicate that for
each case the most efficient of quoted-printable and base64 should be
used.

qp-or-base64 has another effect. It will fold long lines so that
MIME parts may not be broken by MTA. So do quoted-printable and
base64.

Note that it affects body encoding only when a part is a raw forwarded
message (which will be made by gnus-summary-mail-forward with the
arg 2 for example) or is neither the ‘text/*’ type nor the
‘message/*’ type. Even though in those cases, you can override
this setting on a per-message basis by using the encodingMML tag (see section MML Definition).

mm-use-ultra-safe-encoding

When this is non-nil, it means that textual parts are encoded as
quoted-printable if they contain lines longer than 76 characters or
starting with "From " in the body. Non-7bit encodings (8bit, binary)
are generally disallowed. This reduce the probability that a non-8bit
clean MTA or MDA changes the message. This should never be set
directly, but bound by other functions when necessary (e.g., when
encoding messages that are to be digitally signed).

2.5 Charset Translation

During translation from MML to MIME, for each
MIME part which has been composed inside Emacs, an appropriate
charset has to be chosen.

If you are running a non-MULE Emacs, this process is simple: If the
part contains any non-ASCII (8-bit) characters, the MIME charset
given by mail-parse-charset (a symbol) is used. (Never set this
variable directly, though. If you want to change the default charset,
please consult the documentation of the package which you use to process
MIME messages.
See (message)Various Message Variables section ‘Various Message Variables’ in Message Manual, for example.)
If there are only ASCII characters, the MIME charset US-ASCII is
used, of course.

Things are slightly more complicated when running Emacs with MULE
support. In this case, a list of the MULE charsets used in the
part is obtained, and the MULE charsets are translated to
MIME charsets by consulting the table provided by Emacs itself
or the variable mm-mime-mule-charset-alist for XEmacs.
If this results in a single MIME charset, this is used to encode
the part. But if the resulting list of MIME charsets contains more
than one element, two things can happen: If it is possible to encode the
part via UTF-8, this charset is used. (For this, Emacs must support
the utf-8 coding system, and the part must consist entirely of
characters which have Unicode counterparts.) If UTF-8 is not available
for some reason, the part is split into several ones, so that each one
can be encoded with a single MIME charset. The part can only be
split at line boundaries, though—if more than one MIME charset is
required to encode a single line, it is not possible to encode the part.

When running Emacs with MULE support, the preferences for which
coding system to use is inherited from Emacs itself. This means that
if Emacs is set up to prefer UTF-8, it will be used when encoding
messages. You can modify this by altering the
mm-coding-system-priorities variable though (see section Encoding Customization).

The charset to be used can be overridden by setting the charsetMML tag (see section MML Definition) when composing the message.

The encoding of characters (quoted-printable, 8bit, etc.) is orthogonal
to the discussion here, and is controlled by the variables
mm-body-charset-encoding-alist and
mm-content-transfer-encoding-defaults (see section Encoding Customization).

2.6 Conversion

A (multipart) MIME message can be converted to MML
with the mime-to-mml function. It works on the message in the
current buffer, and substitutes MML markup for MIME
boundaries. Non-textual parts do not have their contents in the buffer,
but instead have the contents in separate buffers that are referred to
from the MML tags.

An MML message can be converted back to MIME by the
mml-to-mime function.

These functions are in certain senses “lossy”—you will not get back
an identical message if you run mime-to-mml and then
mml-to-mime. Not only will trivial things like the order of the
headers differ, but the contents of the headers may also be different.
For instance, the original message may use base64 encoding on text,
while mml-to-mime may decide to use quoted-printable encoding, and
so on.

In essence, however, these two functions should be the inverse of each
other. The resulting contents of the message should remain equivalent,
if not identical.

2.7 Flowed text

The Emacs MIME library will respect the use-hard-newlines
variable (see (emacs)Hard and Soft Newlines section ‘Hard and Soft Newlines’ in Emacs Manual) when encoding a message, and the
“format=flowed” Content-Type parameter when decoding a message.

On encoding text, regardless of use-hard-newlines, lines
terminated by soft newline characters are filled together and wrapped
after the column decided by fill-flowed-encode-column.
Quotation marks (matching ‘^>* ?’) are respected. The variable
controls how the text will look in a client that does not support
flowed text, the default is to wrap after 66 characters. If hard
newline characters are not present in the buffer, no flow encoding
occurs.

You can customize the value of the mml-enable-flowed variable
to enable or disable the flowed encoding usage when newline
characters are present in the buffer.

On decoding flowed text, lines with soft newline characters are filled
together and wrapped after the column decided by
fill-flowed-display-column. The default is to wrap after
fill-column.

3. Interface Functions

The mail-parse library is an abstraction over the actual
low-level libraries that are described in the next chapter.

Standards change, and so programs have to change to fit in the new
mold. For instance, RFC2045 describes a syntax for the
Content-Type header that only allows ASCII characters in the
parameter list. RFC2231 expands on RFC2045 syntax to provide a scheme
for continuation headers and non-ASCII characters.

The traditional way to deal with this is just to update the library
functions to parse the new syntax. However, this is sometimes the wrong
thing to do. In some instances it may be vital to be able to understand
both the old syntax as well as the new syntax, and if there is only one
library, one must choose between the old version of the library and the
new version of the library.

The Emacs MIME library takes a different tack. It defines a
series of low-level libraries (‘rfc2047.el’, ‘rfc2231.el’
and so on) that parses strictly according to the corresponding
standard. However, normal programs would not use the functions
provided by these libraries directly, but instead use the functions
provided by the mail-parse library. The functions in this
library are just aliases to the corresponding functions in the latest
low-level libraries. Using this scheme, programs get a consistent
interface they can use, and library developers are free to create
write code that handles new standards.

The following functions are defined by this library:

mail-header-parse-content-type

Parse a Content-Type header and return a list on the following
format:

4. Basic Functions

This chapter describes the basic, ground-level functions for parsing and
handling. Covered here is parsing From lines, removing comments
from header lines, decoding encoded words, parsing date headers and so
on. High-level functionality is dealt with in the first chapter
(see section Decoding and Viewing).

4.1 rfc2045

RFC2045 is the “main” MIME document, and as such, one would
imagine that there would be a lot to implement. But there isn’t, since
most of the implementation details are delegated to the subsequent
RFCs.

So ‘rfc2045.el’ has only a single function:

rfc2045-encode-string

Takes a parameter and a value and returns a ‘PARAM=VALUE’ string.
value will be quoted if there are non-safe characters in it.

4.4 rfc2047

RFC2047 (Message Header Extensions for Non-ASCII Text) specifies how
non-ASCII text in headers are to be encoded. This is actually rather
complicated, so a number of variables are necessary to tweak what this
library does.

The following variables are tweakable:

rfc2047-header-encoding-alist

This is an alist of header / encoding-type pairs. Its main purpose is
to prevent encoding of certain headers.

The keys can either be header regexps, or t.

The values can be nil, in which case the header(s) in question
won’t be encoded, mime, which means that they will be encoded, or
address-mime, which means the header(s) will be encoded carefully
assuming they contain addresses.

rfc2047-charset-encoding-alist

RFC2047 specifies two forms of encoding—Q (a
Quoted-Printable-like encoding) and B (base64). This alist
specifies which charset should use which encoding.

rfc2047-encode-function-alist

This is an alist of encoding / function pairs. The encodings are
Q, B and nil.

rfc2047-encoded-word-regexp

When decoding words, this library looks for matches to this regexp.

rfc2047-encoded-word-regexp-loose

This is a version from which the regexp for the Q encoding pattern of
rfc2047-encoded-word-regexp is made loose.

rfc2047-encode-encoded-words

The boolean variable specifies whether encoded words
(e.g., ‘=?us-ascii?q?hello?=’) should be encoded again.
rfc2047-encoded-word-regexp is used to look for such words.

rfc2047-allow-irregular-q-encoded-words

The boolean variable specifies whether irregular Q encoded words
(e.g., ‘=?us-ascii?q?hello??=’) should be decoded. If it is
non-nil, rfc2047-encoded-word-regexp-loose is used instead
of rfc2047-encoded-word-regexp to look for encoded words.

Those were the variables, and these are this functions:

rfc2047-narrow-to-field

Narrow the buffer to the header on the current line.

rfc2047-encode-message-header

Should be called narrowed to the header of a message. Encodes according
to rfc2047-header-encoding-alist.

rfc2047-encode-region

Encodes all encodable words in the region specified.

rfc2047-encode-string

Encode a string and return the results.

rfc2047-decode-region

Decode the encoded words in the region.

rfc2047-decode-string

Decode a string and return the results.

rfc2047-encode-parameter

Encode a parameter in the RFC2047-like style. This is a substitution
for the rfc2231-encode-string function, that is the standard but
many mailers don’t support it. See section rfc2231.

4.5 time-date

While not really a part of the MIME library, it is convenient to
document this library here. It deals with parsing Date headers
and manipulating time. (Not by using tesseracts, though, I’m sorry to
say.)

These functions convert between five formats: A date string, an Emacs
time structure, a decoded time list, a second number, and a day number.

4.6 qp

This library deals with decoding and encoding Quoted-Printable text.

Very briefly explained, qp encoding means translating all 8-bit
characters (and lots of control characters) into things that look like
‘=EF’; that is, an equal sign followed by the byte encoded as a hex
string.

The following functions are defined by the library:

quoted-printable-decode-region

QP-decode all the encoded text in the specified region.

quoted-printable-decode-string

Decode the QP-encoded text in a string and return the results.

quoted-printable-encode-region

QP-encode all the encodable characters in the specified region. The third
optional parameter fold specifies whether to fold long lines.
(Long here means 72.)

quoted-printable-encode-string

QP-encode all the encodable characters in a string and return the
results.

5. Standards

The Emacs MIME library implements handling of various elements
according to a (somewhat) large number of RFCs, drafts and standards
documents. This chapter lists the relevant ones. They can all be
fetched from http://quimby.gnus.org/notes/.

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of such a section when you modify the Document means that it remains a
section “Entitled XYZ” according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document’s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify
you as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version
if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.
C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
F. Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History,” Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page. If
there is no section Entitled “History” in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.
J. Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on. These may be placed in the “History” section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications,”
Preserve the Title of the section, and preserve in the section all
the substance and tone of each of the contributor acknowledgements
and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section titles.
M. Delete any section Entitled “Endorsements.” Such a section
may not be included in the Modified Version.
N. Do not retitle any existing section to be Entitled “Endorsements”
or to conflict in title with any Invariant Section.
O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements,” provided it contains
nothing but endorsements of your Modified Version by various
parties–for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements,”
and any sections Entitled “Dedications.” You must delete all sections
Entitled “Endorsements.”

COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document’s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements,”
“Dedications,” or “History,” the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License. Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.

FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (C) yearyour name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License.''

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with...Texts.” line with this:

with the Invariant Sections being list their titles, with the
Front-Cover Texts being list, and with the Back-Cover Texts being
list.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.